The invention relates to a servo control, especially for power steering for motor vehicles, with a shaft which serves for actuation and is subdivided axially into an input-side primary-shaft part and an output-side secondary-shaft part. One shaft part projects axially into a bush part coupled fixedly in terms of rotation to the other shaft part and forms with the latter a servo-valve arrangement working in dependence on relative rotations between the shaft parts, in the manner of a rotary-slide arrangement. A torsion bar connects the shaft parts in a rotationally elastic manner and is arranged in an axial bore of the shaft parts so as to leave an annular space free in the one shaft part. A prestressed spring is provided which is effective between the bush part and the one shaft part and the prestress of which seeks to keep the bush part and the one shaft part in a middle position relative to one another.
A corresponding power steering is the subject of British Patent Document GB 2,165,502 A. The prestressed spring determines a minimum torque which has to take effect on the shaft in order to obtain an adjustment of the servo-valve arrangement out of the middle position or neutral position. Moreover, this spring, together with the prestress-free torsion bar, determines the amount of further adjustment of the servo-valve arrangement in the event of torques which increase further.
This arrangement ensures that the servo control or power steering works purely mechanically, that is to say without power assistance, within a range of steering forces which is predetermined by the prestress of the prestressed spring. This guarantees, where a power steering of a motor vehicle is concerned, that the driver acquires a good feel for the steering forces actually effective on the steering wheels of the vehicle.
German Patent Document DE 3,925,992 Al discloses a further servo control or power steering, in which two coaxial shaft parts are coupled to one another positively, but with play, in such a way that limited relative rotations between these parts are possible counter to the resistance of a prestressed leg spring arranged between them. These relative rotations bring about a pivoting stroke of a lever which is arranged between the shaft parts and which thereby actuates the control slide of a servo-valve arranged in the one shaft part. The prestressed leg spring projects with two radial legs into axial slots corresponding to one another in axially mutually overlapping endpieces of the two shaft parts. The mounting of the leg spring is comparatively complicated, since a visual check is possible only to a restricted extent and the lever actuating the control slide of the servo-valve also has to be mounted between the two endpieces.
An object of the invention is to achieve an even further constructive simplification in a servo control of the type mentioned in the introduction.
This object is achieved, according to the invention, in that the one shaft part and the bush part have, at their ends facing the other shaft part, corresponding axial slots open towards the other shaft part, and in that there is arranged as a prestressed spring in the annular space between the torsion bar and the one shaft part a leg spring, the legs of which project into respective corresponding axial slots of the bush part and of the one shaft part and, in the circumferential direction of the shaft axis, have a smaller cross-section than the axial slots and, in the middle position, rest on respective corresponding axial edges of the two axial slots.
As a result of the design according to the invention, the mounting of the leg spring and the coupling of the other shaft part take place on the same end face of the bush part. Accordingly, after the arrangement of the bush part on the one shaft part, that is to say after the mounting of the servo-valve arrangement, the leg spring and the further shaft part can be arranged and connected to the bush part respectively. This results in a mounting sequence which can be carried out in a simple way, whilst the arrangement of the leg spring can be carried out completely with a visual check.
Moreover, it is advantageous that, in this design, the legs of the leg spring can also act as a limitation for the maximum relative rotation between the shaft parts.
According to an expedient embodiment of the invention, the leg spring has a flat characteristic, that is to say the prestress of the leg spring is increased only relatively little in the event of additional elastic deformation of the latter. In contrast, the prestress free torsion bar can have a comparatively steep characteristic, that is to say, in order to achieve an additional torsion of the torsion bar, the torques must increase comparatively sharply.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.